The answer to your question would be that in the sentence above a comma should be placed after "first grade". That is, the sentence would be as follows: Kari and Misha, who have known each other since first grade, are both yearbook editors and photographers.
"Who have known each other since first grade" is a nonessential relative clause. A nonessential relative clause is one that is not necessary. Consequently, the clause can be removed from the sentence without altering the sentence's meaning. When a sentence has a nonessential relative clause, the clause is set apart with commas.